The HIV Preclinical Vaccine Development Master Agreement Program was initiated to enable the NIAID to provide resources to accelerate the development of promising vaccine approaches. Under this program, a senses of vaccine studies have been initiated to evaluate candidate and prototype AIDS vaccines in nonhuman primates. These studies are designed to obtain more information about the efficacy and optimum presentation of candidate HIV vaccines that are in human Phase I clinical trials or proposed for such trials as well as prototype SIV vaccines of the same type as these HIV vaccines. Other vaccine studies will evaluate prototype HIV or SIV vaccines that offer novel presentation systems may be capable of eliciting broadly cross-reactive immune responses, or have shown promising preliminary results. NIAID has initiated collaborative arrangements with researchers who have agreed to provide HIV and SIV vaccines for immunization of nonhuman primates. In all of the vaccine studies, the nonhuman primates will be challenged after immunization to determine if protection from infection or from disease can be obtained. The contractors in Category G of the Master Agreement Program, including the contractor listed above, will evaluate the ability of antibodies produced in the nonhuman primates during vaccine immunization to neutralize the infectivity of SIV and/or HIV, using in vitro assays to assess this activity.